The invention relates to a method of increasing the tensile strength, breaking strength and flexural strength of colored leads and to colored pencils containing such leads.
Colored leads are leads containing pigments, color lakes or dyes, which are used for colored pencils and cosmetics pencils. On the one hand, colored leads should be easy to apply, but must be solid enough to enable processing. Thus, when used in cosmetics pencils, for example, they should be able to be applied gently and softly to delicate areas of skin and readily and intensively impart color upon application. For this reason, they do not generally have a crystalline structure, but exhibit thixotropic behavior and desirably have only a low breaking strength. However, as result of this their tensile strength and flexural strength is also low, which is disadvantageous. During shaping, the proportion of waste is high since the leads readily break when removed from the mold or when subjected to mechanical stress. Particularly in the case of leads which have an unfavorable length to diameter ratio, for example a diameter of from 2 to 6 mm for a length up to 25 to 50 mm, the risk of breakage is high.
The low strength, in particular tensile strength and flexural strength, has proven particularly disadvantageous if thin colored leads having a diameter of less than 6 mm are to be inserted into a rotating pencil where, inserted into a rotary mechanism, they can be rotated out and back, but are not mechanically supported.
In the field of cosmetics pencils, so called xe2x80x9clinersxe2x80x9dxe2x80x94especially for eyeliner pencils and lipliner pencils, for examplexe2x80x94which are supplied inserted into rotary mechanisms, are popular. These liners do not have to be sharpened and the lead, when it is not being used, is protected in the liner sleeve.
Colored leads are usually produced by pouring a homogenized basic mass based on fat/wax mixtures, which, apart from the color-imparting pigments, may comprise further additives, into a mold, or extruding it. For use, the lead is either embedded in wood, poured into preshaped sleeves of wood or plastic or the like, or inserted into a rotary mechanism.
If casting molds are used, considerable forces of adhesion arise in the cooled mold, which, despite the use of release agents, readily leads to tearing of the castings and thus to wastage. Moreover, extensive cleaning operations in the casting plant are required. The insertion of the colored leads into a rotary mechanism and the rotating out of the leads also frequently leads to breakage.
It is therefore an object of the invention to overcome the disadvantages associated hitherto with colored leads and to improve known pencil compositions such that the tensile strength and flexural strength is increased, such that even thin leads can be cast and extruded without removal of the leads from the casting mold or insertion of the leads into a rotary mechanism leading to breakage or damage of the leads. However, at the same time the good and desired properties, in particular the good application properties, of such leads should not be changed detrimentally.
The foregoing object is achieved by providing, a method of increasing the tensile strength, breaking strength and flexural strength of colored leads which is characterized in that 1 to 50% by weight of alkyl- and/or hydroxyalkylcellulose whose alkyl radicals may be straight-chain or branched and preferably have 2 to 10 carbon atoms, which is soluble in organic solvents are added to the lead mass.